Internet Engineering Task Force (IETF) D. Kuegler
Request for Comments: 6631 BSI
Category: Experimental Y. Sheffer
ISSN: 2070-1721 Porticor
June 2012
Password Authenticated Connection Establishment
with the Internet Key Exchange Protocol version 2 (IKEv2)
Abstract
The Internet Key Exchange protocol version 2 (IKEv2) does not allow
secure peer authentication when using short credential strings, i.e.,
passwords. Several proposals have been made to integrate password-
authentication protocols into IKE. This document provides an
adaptation of Password Authenticated Connection Establishment (PACE)
to the setting of IKEv2 and demonstrates the advantages of this
integration.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for examination, experimental implementation, and
evaluation.
This document defines an Experimental Protocol for the Internet
community. This document is a product of the Internet Engineering
Task Force (IETF). It represents the consensus of the IETF
community. It has received public review and has been approved for
publication by the Internet Engineering Steering Group (IESG). Not
all documents approved by the IESG are a candidate for any level of
Internet Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6631.
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Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................3
1.1. Terminology ................................................4
2. Overview ........................................................5
3. Protocol Sequence ...............................................6
3.1. The IKE_SA_INIT Exchange ...................................6
3.2. The IKE_AUTH Exchange, Round #1 ............................7
3.3. The IKE_AUTH Exchange, Round #2 ............................7
3.4. Public Key Validation ......................................8
3.5. Creating a Long-Term Shared Secret .........................9
3.6. Using the Long-Term Shared Secret .........................11
4. Encrypting and Mapping the Nonce ...............................11
4.1. Encrypting the Nonce ......................................11
4.2. Mapping the Nonce .........................................12
4.2.1. Modular Diffie-Hellman .............................13
4.2.2. Elliptic Curve Diffie-Hellman ......................13
5. Protocol Details ...............................................13
5.1. Password Processing .......................................13
5.2. The SECURE_PASSWORD_METHODS Notification ..................14
5.3. The PSK_PERSIST Notification ..............................15
5.4. The PSK_CONFIRM Notification ..............................15
5.5. The GSPM(ENONCE) Payload ..................................15
5.6. The KE (KEi2/KEr2) Payloads in PACE .......................16
5.7. PACE and Session Resumption ...............................16
6. Security Considerations ........................................16
6.1. Credential Security Assumptions ...........................16
6.2. Vulnerability to Passive and Active Attacks ...............16
6.3. Perfect Forward Secrecy ...................................17
6.4. Randomness ................................................17
6.5. Identity Protection .......................................17
6.6. Denial of Service .........................................17
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6.7. Choice of Encryption Algorithms ...........................17
6.8. Security Model and Security Proof .........................18
6.9. Long-Term Credential Storage ..............................18
7. IANA Considerations ............................................19
8. Acknowledgments ................................................19
9. References .....................................................19
9.1. Normative References ......................................19
9.2. Informative References ....................................20
Appendix A. Protocol Selection Criteria ...........................22
A.1. Security Criteria ..........................................22
A.2. Intellectual Property Criteria .............................22
A.3. Miscellaneous Criteria .....................................22
Appendix B. Password Salting ......................................23
B.1. Solving the Asymmetric Case with Symmetric Cryptography ....24
B.2. Solving the Fully Symmetric Case with Asymmetric
Cryptography ...............................................25
B.3. Generation of a Strong, Long-Term, Shared Secret ...........26
1. Introduction
PACE [TR03110] is a security protocol that establishes a mutually
authenticated (and encrypted) channel between two parties based on
weak (short) passwords. PACE provides strong session keys that are
independent of the strength of the password. PACE belongs to a
family of protocols often referred to as Zero-Knowledge Password
Proof (ZKPP) protocols, all of which amplify weak passwords into
strong session keys. This document describes the integration of PACE
into IKEv2 [RFC5996] as a new authentication mode, analogous to the
existing certificate and Pre-Shared Key (PSK) authentication modes.
Some of the advantages of our approach, compared to the existing
IKEv2, include the following:
o The current best practice to implement password authentication in
IKE involves certificate-based authentication of the server plus
some Extensible Authentication Protocol (EAP) method to
authenticate the client. This involves two non-trivial
infrastructure components (PKI and EAP/AAA). Moreover,
certificate authentication is hard to get right and often depends
on unreliable user behavior for its security.
o Alternatively, native IKEv2 shared secret authentication can be
used with passwords. However, this usage is insecure;
specifically, it is vulnerable to active attackers.
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o Some newer EAP methods can be used for mutual authentication and,
combined with [RFC5998], can be well integrated into IKEv2. This
is certainly an option in some cases, but the current proposal may
be simpler to implement.
Compared to other protocols aiming at similar goals, PACE has several
advantages. PACE was designed to allow for a high level of
flexibility with respect to cryptographic algorithms; e.g., it can be
implemented based on Modular Diffie-Hellman as well as Elliptic Curve
Diffie-Hellman without any restrictions on the mathematical group to
be used, other than the requirement that the group be
cryptographically secure. The protocol itself is also proven to be
cryptographically secure [PACEsec]. The PACE protocol is currently
used in an international standard for digital travel documents
[ICAO].
The integration aims at keeping IKEv2 unchanged as much as possible;
e.g., the mechanisms used to establish Child security associations
(SAs) as provided by IKEv2 would be maintained with no change.
The Password-Authenticated Key Exchange (PAKE) framework document
[RFC6467] defines a set of payloads for different types of PAKE
methods within IKEv2. This document reuses this framework. Note
that the current document is self-contained; i.e., all relevant
payloads and semantics are redefined here.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
The following notation is used in this document:
E() Symmetric encryption
D() Symmetric decryption
KA() Key agreement
Map() Mapping function
Pwd Shared password
SPwd Stored password
KPwd Symmetric key derived from a password Pwd
G Static group generator
GE Ephemeral group generator
ENONCE Encrypted nonce
PKEi Ephemeral public key of the initiator
SKEi Ephemeral secret key of the initiator
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PKEr Ephemeral public key of the responder
SKEr Ephemeral secret key of the responder
AUTH Authentication payload
Any other notation used here is defined in [RFC5996].
2. Overview
At a high level, the following steps are performed by the initiator
and the responder. They result in a two-round IKE_AUTH exchange,
described in Section 3 below.
1. The initiator randomly and uniformly chooses a nonce s, encrypts
the nonce using the password, and sends the ciphertext
ENONCE = E(KPwd, s)
to the responder. The responder recovers the plaintext nonce s
with the help of the shared password Pwd.
2. The nonce s is mapped to an ephemeral generator
GE = G^s * SASharedSecret,
where G is the generator of the selected Modular Exponential
(MODP) group and SASharedSecret is a shared secret that has been
generated in the IKE_SA_INIT step.
3. Both the initiator and the responder each calculate an ephemeral
key pair
(SKEi, PKEi = GE^SKEi) and (SKEr, PKEr=GE^SKEr),
respectively, based on the ephemeral generator GE, and exchange
the public keys.
4. Finally, they compute the shared secret
PACESharedSecret = PKEi^SKEr = PKEr^SKEi
and generate, exchange, and verify the IKE authentication token
AUTH using the shared secret PACESharedSecret.
The encryption function E() must be carefully chosen to prevent
dictionary attacks that would otherwise allow an attacker to recover
the password. Those restrictions are described in Section 4.1.
Details on the mapping function, including the elliptic curve
variant, can be found in Section 4.2.
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To avoid the risks inherent in storing a short password (e.g., the
fact that passwords are often reused for different applications),
this protocol allows the peers to jointly convert the password into a
cryptographically stronger shared secret. This shared secret can
then be stored by both peers, in lieu of the original password or its
salted variants.
3. Protocol Sequence
The protocol consists of three round trips -- an IKE_SA_INIT exchange
and a 2-round IKE_AUTH exchange -- as shown in the next figure. An
optional Informational exchange may follow (see Section 3.5).
Initiator Responder
--------- ---------
IKE_SA_INIT:
HDR, SAi1, KEi, Ni, N(SECURE_PASSWORD_METHODS) ->
RFC 6631 IKEv2 with PACE June 2012
If PACE was chosen, the algorithms negotiated in SAi1 and SAr1 are
also used for the execution of PACE, i.e., the key agreement protocol
(Modular Diffie-Hellman or Elliptic Curve Diffie-Hellman), the group
to be used, and the encryption algorithm.
3.2. The IKE_AUTH Exchange, Round #1
This is the first part of the PACE authentication of the peers. This
exchange MUST NOT be used unless both peers indicated support of this
protocol.
The initiator selects a random nonce s and encrypts it to form ENONCE
using the password Pwd, as described in Section 4.1. Then, the
initiator maps the nonce to an ephemeral generator GE of the group as
described in Section 4.2, chooses randomly and uniformly an ephemeral
key pair (SKEi,PKEi) based on the ephemeral generator, and finally
generates the payloads GSPM(ENONCE) containing the encrypted nonce
and KEi2 containing the ephemeral public key.
The responder decrypts the received encrypted nonce s = D(KPwd,
ENONCE), performs the mapping, and randomly and uniformly chooses an
ephemeral key pair (SKEr,PKEr) based on the ephemeral generator GE.
The responder generates the KEr2 payload containing the ephemeral
public key.
The request is equivalent to the IKE_AUTH request in a normal IKEv2
exchange; i.e., any payload that is valid in an IKE_AUTH request is
valid (with the same semantics) in this round's request. In
particular, certificate-related payloads are allowed, even though
their use may not be practical within this mode.
3.3. The IKE_AUTH Exchange, Round #2
This is the second part of the PACE authentication of the peers.
The initiator and the responder calculate the shared secret
PACESharedSecret
PACESharedSecret = KA(SKEi, PKEr, GE) = KA(SKEr, PKEi, GE),
where KA denotes the Diffie-Hellman key agreement, e.g., (for MODP
groups), modular exponentiation. Then, they calculate the
authentication tokens AUTHi and AUTHr.
The initiator calculates
AUTHi = prf(prf+(Ni | Nr, PACESharedSecret),
| PKEr)
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See Section 2.15 of [RFC5996] for the definition of signed octets.
The responder calculates
AUTHr = prf(prf+(Ni | Nr, PACESharedSecret),
| PKEi)
Both AUTH payloads MUST indicate as their authentication method the
Generic Secure Password Authentication Method [RFC6467], whose value
is 12. The authentication tokens are exchanged, and each of them
MUST be verified by the other party. The behavior when this
verification fails is unchanged from [RFC5996].
Each of the peers MAY generate a long-term credential at this point,
after it has verified the opposite peer's identity. The shared
secret is
LongTermSecret = prf(Ni | Nr, "PACE Generated PSK" |
PACESharedSecret),
where the literal string is ASCII-encoded, with no zero terminator.
The generated secret MUST be persisted to stable memory before
sending the response. See Section 3.5 for more details about this
facility.
This round's response is equivalent to the IKE_AUTH response in a
normal IKEv2 exchange; i.e., any payload that is valid in an IKE_AUTH
response is valid (with the same semantics) in the second round's
response.
Following authentication, all temporary values MUST be deleted by the
peers, including in particular s, the ephemeral generator, the
ephemeral key pairs, and PACESharedSecret.
3.4. Public Key Validation
The security of the protocol relies on the entanglement of a weak
password with cryptographically strong shared secrets, SASharedSecret
and PACESharedSecret, mutually and randomly generated by the
initiator and the responder. If an attacker can influence the
randomness of those shared secrets, the confidentiality of the
password may be directly affected.
Implementations MUST therefore verify that the shared secrets
SASharedSecret and PACESharedSecret are random elements of the group
generated by G to prevent small subgroup attacks. This can be
achieved by a validation of the public keys (i.e., KEi, PKEi, and
KEr, PKEr).
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First of all, each party MUST check that the public keys PKEi, PKEr,
KEi, and KEr differ. Otherwise, it MUST abort the protocol.
For each received public key PK, the following tests SHOULD be
performed. Any failure in the validation MUST be interpreted as an
attack, and the protocol SHALL be aborted.
o Verify that PK is an element of the Diffie-Hellman Group.
* For Modular Diffie-Hellman, check that PK lies within the
interval [2,p-2].
* For Elliptic Curve Diffie-Hellman, check that PK is a point on
the Elliptic Curve and not the point at infinity.
o Verify that PK is an element of the cryptographic subgroup of
order q.
* For Modular Diffie-Hellman, check that PK^q = 1 (mod p).
* For Elliptic Curve Diffie-Hellman, check that q * PK = 0.
Note that for most of the MODP groups, the order q = (p-1)/2. This
applies in particular to the standard groups #2, #5, and #14,
commonly used in IKE. For ECP and MODP groups not based on safe
primes, the order q is explictly stated in the parameters.
As an alternative to the public key validation, the compatible
cofactor exponentiation/multiplication may be used, which is often
more efficient but requires changes to the implementation of the key
agreement. Details on the implementation can be found in [RFC2785]
and in [TR03111] for Modular Diffie-Hellman and Elliptic Curve
Diffie-Hellman, respectively.
3.5. Creating a Long-Term Shared Secret
To reduce the time that the peers store a hashed password, it is
RECOMMENDED that the password be replaced by a dedicated shared
secret, according to the method described in this section. See
Appendix B for more discussion of the security threats involved.
Both peers generate the value LongTermSecret during round #2 of
IKE_AUTH, as shown above. Later on, they exchange a PERSIST_PSK
notification. Assume that both peers support this mechanism (e.g.,
the IKE implementation is able to modify its own credential store).
Then, each of the peers, when receiving the notification, permanently
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deletes the stored password and replaces it with LongTermSecret.
These credentials are stored in the Peer Authorization Database (PAD)
[RFC4301] and are associated with the identity of the opposite peer.
This solution is designed as a two-phase commitment, so that failure
at any time cannot result in the peers not having any shared secret.
Initiator Responder
--------- ---------
IKE_AUTH round #2:
HDR, SK{..., N(PSK_PERSIST)} ---------->
Responder computes and stores PSK
Responder deletes the short password
RFC 6631 IKEv2 with PACE June 2012
3.6. Using the Long-Term Shared Secret
The LongTermSecret MUST be used as a regular IKE Pre-Shared Key
(PSK), rather than with PACE or any other password-based
authentication method.
Normally, at the completion of this protocol, both peers will have
either a shared password or a shared PSK. The protocol is designed
so that the peers will have a shared credential, regardless of any
protocol failures. However, in some failure cases, the initiator may
find itself with both a short password and a PSK for a particular
peer. In that case, it MUST first try to authenticate with a
password and, upon success, MUST attempt to convert it to a PSK. If
password authentication fails, it MUST use the PSK and upon
successful setup of the IKE SA MUST permanently delete the password.
4. Encrypting and Mapping the Nonce
4.1. Encrypting the Nonce
The shared password is not used as is. Instead, it SHOULD be
converted into a "stored password" SPwd, so that the plaintext
password does not need to be stored for long periods. SPwd is
defined as
SPwd = prf("IKE with PACE", Pwd),
where the literal string consists of ASCII characters with no zero
terminator. If the negotiated pseudorandom function (prf) requires a
fixed-size key, the literal string is either truncated or padded with
zero octets on the right, as needed. Multiple copies of SPwd MAY be
stored, if the prf function is not known in advance.
KPwd = prf+(Ni | Nr, SPwd),
where Ni and Nr are the regular IKE nonces, stripped of any headers.
If the negotiated prf takes a fixed-length key and the lengths of Ni
and Nr do not add up to that length, half the bits must come from Ni
and half from Nr, taking the first bits of each. "prf+" is defined
in Section 2.13 of [RFC5996]. The length of KPwd is determined by
the key length of the negotiated encryption algorithm.
A nonce s is randomly selected by the initiator (see Section 6.4 for
additional considerations). The length of s MUST be exactly
32 octets.
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KPwd is now used with the encryption transform to encrypt the nonce:
ENONCE = E(KPwd, s)
If an Initialization Vector (IV) is required by the cipher, it MUST
be included in the GSPM(ENONCE) payload. It is RECOMMENDED that the
IV be chosen both randomly and uniformly distributed, even though
this condition is not necessary for the cryptographic security of the
protocol.
Note: Padding MUST NOT be used when encrypting the nonce. The size
of the nonce has been chosen such that it can be encrypted with block
ciphers having block sizes of 32, 64, and 128 bits without any
padding.
If an authenticated encryption cipher [RFC5282] has been negotiated
for the IKE SA, it MUST NOT be used as is because such use would be
vulnerable to dictionary attacks. Instead, the corresponding
unauthenticated mode MUST be used. All Galois/Counter Mode (GCM) and
all Counter with CBC-MAC (CCM) encryption algorithms are mapped to
the corresponding counter-mode algorithm. For example, if the
negotiated encryption algorithm (Transform Type 1) is "AES-GCM with
an 8-octet Integrity Check Value (ICV)", then ENCR_AES_CTR (with the
same key length) is used to encrypt the nonce. If such a mapping
does not exist for a particular cipher, then it MUST NOT be used
within the current protocol.
4.2. Mapping the Nonce
The mapping is based on a second anonymous Diffie-Hellman key
agreement protocol to create a shared secret that is used together
with the exchanged nonce to calculate a common secret generator of
the group.
While in [TR03110] the generation of the shared secret is part of the
mapping, in the setting of IKEv2, a shared secret SASharedSecret has
already been generated as part of the IKE_SA_INIT step. Using the
notation of [RFC5996],
SASharedSecret = g^ir
Let G and GE be the generator of the negotiated Diffie-Hellman group,
and the calculated ephemeral generator, respectively. The following
subsections describe the mapping for different Diffie-Hellman
variants.
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4.2.1. Modular Diffie-Hellman
The function Map:G->GE is defined as GE = G^s * SASharedSecret.
Note that the protocol will fail if G^s = 1/SASharedSecret. If s is
chosen randomly, this event occurs with negligible probability. In
implementations that detect such a failure, the initiator SHOULD
choose s again.
4.2.2. Elliptic Curve Diffie-Hellman
The function Map:G->GE is defined as GE = s*G + SASharedSecret.
Note that the protocol will fail if s*G = -SharedSecret. If s is
chosen randomly, this event occurs with negligible probability. In
implementations that detect such a failure, the initiator SHOULD
choose s again.
5. Protocol Details
5.1. Password Processing
The input password string SHOULD be processed according to the rules
of the [RFC4013] profile of [RFC3454]. A password SHOULD be
considered a "stored string" per [RFC3454]; therefore, unassigned
code points are prohibited. The output is the binary representation
of the processed UTF-8 character string. Prohibited output and
unassigned codepoints encountered in SASLprep preprocessing SHOULD
cause a preprocessing failure, and the output SHOULD NOT be used. A
compliant implementation MUST NOT apply any other form of processing
to the input password, other than as described in this section.
See Section 3 of [RFC4013] for examples of SASLprep processing.
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5.2. The SECURE_PASSWORD_METHODS Notification
[RFC6467] defines a new type of Notify payload to indicate support
for Secure Password Methods (SPMs) in the IKE_SA_INIT exchange. The
SPM Notify payload is defined as follows:
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Payload |C| RESERVED | Payload Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Protocol ID | SPI Size | Notify Message Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Security Parameter Index (SPI) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Notification Data ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: SECURE_PASSWORD_METHODS Payload Structure
The Protocol ID is zero, and the SPI Size is also zero, indicating
that the SPI field is empty. The Notify Message Type is
SECURE_PASSWORD_METHODS (value 16424).
The Notification Data contains the list of the 16-bit secure password
method numbers:
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Secure Password Method #1 | Secure Password Method #2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Secure Password Method #3 | ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: SECURE_PASSWORD_METHODS Payload Data
For the current method, the list of proposed methods MUST include the
value PACE (1).
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5.3. The PSK_PERSIST Notification
This document defines the PSK_PERSIST notification type, whose value
is 16425. This notification MUST be sent with no data. However, for
future extensibility, the receiver MUST ignore any notification data
if such data is present.
5.4. The PSK_CONFIRM Notification
This document defines the PSK_CONFIRM notification type, whose value
is 16426. This notification MUST be sent with no data. However, for
future extensibility, the receiver MUST ignore any notification data
if such data is present.
5.5. The GSPM(ENONCE) Payload
This protocol defines the ENONCE (encrypted nonce) payload, which
reuses the Generic SPM (GSPM) payload type [RFC6467] (value 49). Its
format is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Payload |C| RESERVED | Payload Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PACE-RESERVED | Initialization Vector |
+-+-+-+-+-+-+-+-+ +
| (optional, length depends on the encryption algorithm) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Encrypted Nonce ~
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
~ |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: ENONCE Payload Structure
See Section 4.1 for further details about the encrypted nonce. Note
that the protocol -- and in particular this payload's format -- does
not support any padding of the encrypted data.
The PACE-RESERVED field must be sent as zero, and it must be rejected
by the receiver if it is not 0.
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5.6. The KE (KEi2/KEr2) Payloads in PACE
PACE reuses the Key Exchange (KE) payload for its Diffie-Hellman
exchange, with the new payloads being sent within the IKE_AUTH
exchange. Since only one Diffie-Hellman group is negotiated, the
group denoted by these payloads MUST be identical to the one used in
the "regular" KE payloads in IKE_SA_INIT.
5.7. PACE and Session Resumption
A session resumption [RFC5723] ticket may be requested during the
IKE_AUTH exchange. The request MUST be sent in the request of the
first round, and any response MUST be sent in the response of the
second one.
PACE should be considered an "authentication method", in the sense of
Section 5 of [RFC5723], which means that its use MUST be noted in the
protected ticket. The format of the ticket is not standardized;
however, it is RECOMMENDED that this indication distinguish between
the different secure password authentication methods defined for IKE.
Note that even if the initial authentication used PACE and its
extended IKE_AUTH, session resumption will still include the normal
IKE_AUTH exchange.
6. Security Considerations
A major goal of this protocol has been to maintain the level of
security provided by IKEv2. What follows is an analysis of this
protocol. The reader is referred to [RFC5996] for the generic IKEv2
security considerations.
6.1. Credential Security Assumptions
This protocol makes no assumption on the strength of the shared
credential. Best common practices regarding minimal password length,
use of multiple character classes, etc. SHOULD be followed.
6.2. Vulnerability to Passive and Active Attacks
The protocol is secure against both passive and active attackers.
See Section 6.8 for a security proof.
While not attacking the cryptography, an attacker can still perform a
standard password-guessing attack. To mitigate such attacks, an
implementation MUST include standard protections, such as rate-
limiting the number of allowed password-guessing attempts, possibly
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locking identities out after a certain number of failed attempts,
etc. Note that the protocol is symmetric; therefore, this guidance
applies to client-side implementations as well.
6.3. Perfect Forward Secrecy
The key derivation for the IKE SA and any Child SAs is performed as
part of IKEv2 and remains unchanged. It directly follows that
perfect forward security is provided independent of the
authentication additionally performed by PACE.
6.4. Randomness
The security of this protocol depends on the quality generation of
random quantities; see Section 5 of [RFC5996] for more details.
Specifically, any deviation from randomness of the nonce s might
compromise the password. Therefore, it is strongly RECOMMENDED that
the initiator pass the raw random material through a strong prf to
ensure the statistical qualities of the nonce.
6.5. Identity Protection
This protocol is identical to IKEv2 in the quality of identity
protection it provides. Both peers' identities are secure from
passive attackers, and both peers' identities are exposed to active,
man-in-the-middle attackers.
6.6. Denial of Service
We are not aware of any new denial-of-service attack vector enabled
by this protocol.
6.7. Choice of Encryption Algorithms
Any transforms negotiated for IKEv2 may be used by this protocol.
Please refer to Section 4.1 for the considerations regarding
authenticated encryption ("combined mode") algorithms.
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6.8. Security Model and Security Proof
PACE is cryptographically proven secure in [PACEsec] in the model of
Bellare, Pointcheval, and Rogaway [BPRmodel]. The setting in which
PACE is proven secure is, however, slightly different from the
setting used in IKEv2. The differences are described in the
following:
o Part of the mapping is already performed within IKEv2 before PACE
is started. This rearrangement does not affect the proof, as the
resulting PACESharedSecret remains close to uniformly distributed
in the group generated by G.
o The keys for the IKE SA and any Child SAs are already generated
within IKEv2 before PACE is started. While those session keys
could also be derived in PACE, only the keys for the
authentication token are considered in the proof, which explicitly
recommends a separate key for this purpose.
o IKEv2 allows the negotiation of a stream cipher for PACE, while
the proven variant always uses a block cipher. The ideal cipher
is replaced in the proof by a lazy-sampling technique that is
similarly applicable to the stream-cipher-based construction.
The differences in the setting therefore have no impact on the
validity of the proof.
6.9. Long-Term Credential Storage
This protocol does not require that peers store the plaintext
password. Instead, the value SPwd SHOULD be stored by both peers.
In addition, the protocol allows both peers to replace the password
by a crypto-strength shared secret. This solution improves the
system's security (since passwords are often used for multiple
applications), but at the cost of implementation complexity. In
particular, if this optional mechanism is to be used, the credential
database would need to be writable by the key management subsystem.
See Appendix B for alternatives to this approach.
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7. IANA Considerations
IANA has allocated the following values:
o A PACE value of 1 from the "IKEv2 Secure Password Methods"
registry [RFC6467].
o A PSK_PERSIST value of 16425 and a PSK_CONFIRM value of 16426 from
the "IKEv2 Notify Message Types - Status Types" registry. We note
that these notification types are generic and that other password
authentication methods may also choose to use them.
8. Acknowledgments
We would like to thank Dan Harkins for pointing out a security issue
with our use of combined-mode algorithms in a previous version of the
protocol. We thank Tero Kivinen for his generic framework document,
and for a thorough and fruitful review. Hugo Krawczyk proposed that
the password be amplified into a persistent shared secret.
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2785] Zuccherato, R., "Methods for Avoiding the "Small-
Subgroup" Attacks on the Diffie-Hellman Key Agreement
Method for S/MIME", RFC 2785, March 2000.
[RFC3454] Hoffman, P. and M. Blanchet, "Preparation of
Internationalized Strings ("stringprep")", RFC 3454,
December 2002.
[RFC4013] Zeilenga, K., "SASLprep: Stringprep Profile for User
Names and Passwords", RFC 4013, February 2005.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol Version 2 (IKEv2)",
RFC 5996, September 2010.
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9.2. Informative References
[BPRmodel] Bellare, M., Pointcheval, D., and P. Rogaway,
"Authenticated Key Exchange Secure against Dictionary
Attacks", EUROCRYPT 2000, LNCS 1807, pp. 139-155,
Springer-Verlag, 2000, .
[ICAO] ISO/IEC JTC1 SC17 WG3/TF5 for the International Civil
Aviation Organization (ICAO), "Supplemental Access
Control for Machine Readable Travel Documents",
Version 1.01, November 2010.
[IKEv2-CONS] Harkins, D., "Password-Based Authentication in IKEv2:
Selection Criteria and Considerations", Work
in Progress, October 2010.
[PACEsec] Bender, J., Fischlin, M., and D. Kuegler, "Security
Analysis of the PACE Key-Agreement Protocol",
LNCS 5735, pp. 33-48, Springer-Verlag (the extended
abstract appeared in Information Security Conference
(ISC) 2009), December 2009,
.
[RFC5282] Black, D. and D. McGrew, "Using Authenticated
Encryption Algorithms with the Encrypted Payload of the
Internet Key Exchange version 2 (IKEv2) Protocol",
RFC 5282, August 2008.
[RFC5723] Sheffer, Y. and H. Tschofenig, "Internet Key Exchange
Protocol Version 2 (IKEv2) Session Resumption",
RFC 5723, January 2010.
[RFC5998] Eronen, P., Tschofenig, H., and Y. Sheffer, "An
Extension for EAP-Only Authentication in IKEv2",
RFC 5998, September 2010.
[RFC6467] Kivinen, T., "Secure Password Framework for Internet
Key Exchange Version 2 (IKEv2)", RFC 6467,
December 2011.
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[TR03110] BSI, "TR-03110, Advanced Security Mechanisms for
Machine Readable Travel Documents, Part 2 - Extended
Access Control Version 2 (EACv2), Password
Authenticated Connection Establishment (PACE), and
Restricted Identification (RI)", Version 2.10,
March 2012.
[TR03111] BSI, "TR-03111, Elliptic Curve Cryptography",
Version 1.11, April 2009.
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Appendix A. Protocol Selection Criteria
To support the selection of a password-based protocol for inclusion
in IKEv2, a number of criteria are provided in [IKEv2-CONS]. In the
following sections, those criteria are applied to the PACE protocol.
A.1. Security Criteria
SEC1: PACE is a zero-knowledge protocol.
SEC2: The protocol supports perfect forward secrecy and is resistant
to replay attacks.
SEC3: The identity protection provided by IKEv2 remains unchanged.
SEC4: Any cryptographically secure Diffie-Hellman group can be used.
SEC5: The protocol is proven secure in the Bellare-Pointcheval-
Rogaway model.
SEC6: Strong session keys are generated.
SEC7: A transform of the password can be used instead of the
password itself.
A.2. Intellectual Property Criteria
IPR1: The first version of [TR03110] was published on May 21, 2007.
IPR2: BSI has developed PACE aiming to be free of patents. BSI has
not applied for a patent on PACE.
IPR3: The protocol itself is believed to be free of IPR.
A.3. Miscellaneous Criteria
MISC1: One additional exchange is required.
MISC2: The protocol requires the following operations per entity:
* one key derivation from the password,
* one symmetric encryption or decryption,
* one multi-exponentiation for the mapping,
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* one exponentiation for the key pair generation,
* one exponentiation for the shared secret calculation, and
* two symmetric authentications (generation and
verification).
MISC3: The performance is independent of the type/size of password.
MISC4: Internationalization of character-based passwords is
supported.
MISC5: The protocol uses the same group as that negotiated for
IKEv2.
MISC6: The protocol fits into the request/response nature of IKE.
MISC7: The password-based symmetric encryption must be additionally
negotiated.
MISC8: Neither trusted third parties nor clock synchronization are
required.
MISC9: Only general cryptographic primitives are required.
MISC10: Any secure variant of Diffie-Hellman (e.g., Modular or
Elliptic Curve) can be used.
MISC11: The protocol can be implemented easily based on existing
cryptographic primitives.
Appendix B. Password Salting
This protocol requires that passwords not be stored in plaintext.
Instead, we store a hash of the password with a fixed hash. This
value is then used in the ZKPP protocol, replacing the original
password and acting as a "password equivalent". The main benefit of
this solution is that a system administrator or an undetermined
attacker does not get immediate access to the passwords. We believe
this is sufficiently secure for the main usage scenario of the
protocol.
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However, the common practice of password salting is clearly more
powerful, and this appendix presents a few ideas on how password
salting can be applied and/or adapted to fit into a symmetric
protocol such as IKE. First, let us list the threats that we expect
salting to handle, as well as the non-threats:
o The plain password should not be visible to a casual onlooker, as
noted above. It is assumed that very often the same password is
used for multiple applications, and so a password exposed allows
an attacker a starting point for further attacks.
o An attacker must not be able to construct lookup tables (such as
the famous "rainbow tables") that enable her to discover the plain
password.
o IKE is a symmetric protocol, in the sense that any of the peers
might initiate an IKE exchange to another peer. As a result, all
peers must have stored credentials (passwords or password
equivalents) that would enable them to set up an IKE exchange.
So, an attacker that reaches the credential store would in fact be
able to impersonate IKE to another peer. We believe that this
reduces, but does not invalidate, the importance of salting,
because of the other threats that remain.
Below we present different scenarios and solutions that support
password salting in this setting.
We assume that each credential is used to authenticate exactly two
peers to one another; i.e., (as per the best practice), group
credentials are not allowed.
B.1. Solving the Asymmetric Case with Symmetric Cryptography
Despite the protocol's symmetry, there are use cases that are
somewhat asymmetric. Consider the case of an organization that
consists of a headquarters and branches, using a hub-and-spoke
architecture. Communication sessions can be initiated by the center
or by any of the branches, but only the center holds a large
credential database.
Here it would be possible to use traditional password salting,
stored password = hash(salt, password),
where the hash function is a symmetric hash (e.g., HMAC-SHA-256,
using the salt as its key), and the salt is picked at random for each
password. The salt would need to be sent in the first exchange of
the protocol, regardless of which side initiates the session. Unlike
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the normal use of salted passwords, here it is the stored password,
rather than the original password, that is used by the follow-on ZKPP
protocol.
B.2. Solving the Fully Symmetric Case with Asymmetric Cryptography
For the fully symmetric case, we propose a salting method based on a
commutative one-way function. This is essentially a novel variant of
the RSA protocol. Using this solution, all protocol peers can store
the password in a salted form.
The implementation proposed here requires a composite number n that
is common to all peers. The composite number n can be generated by a
trusted (third) party as n = p * q, where p and q are strong primes
(i.e., p = 2 * p' + 1 and q = 2 * q' + 1, where p' and q' are also
primes), and the trusted party promises not to retain a copy of the
primes. Alternatively, n can be chosen randomly and tested for
"small" prime factors. In the latter case, it is certainly not
guaranteed that n is composed of only two primes. While this has the
advantage that no one knows the factorization of n, the disadvantage
is that n is likely to be significantly easier to factor.
Each peer then chooses a public encryption key "e". In a simple
implementation, the encryption key is generated randomly by each
peer, picking a different value for each of the passwords that it
stores.
Note that although the pair (n,e) is similar to an RSA public key,
the usual rules for generating "e" for the RSA protocol do not apply
here, and a random "e" is sufficient. The password is hashed by a
symmetric hash function H (e.g., SHA-256). Each peer i stores the
two values
e_i, H(P)^e_i (mod n),
where P is the original password. The values e_i are exchanged by
the peers before the ZKPP protocol commences (in IKEv2-PACE, this
would be in IKE_SA_INIT), and the following value is used in the ZKPP
protocol run that follows, in lieu of the original password:
H(P) ^ (e_i * e_j) (mod n).
This transformation is used as a salting mechanism only, and the
salted values themselves are never sent on the wire.
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This scheme can be enhanced by basing the value "e" on each peer's
identity (IDi, IDr), e.g., making it a simple hash of the identity.
This eliminates the need to send "e" explicitly and additionally
binds the identity of the peer with its secret.
B.3. Generation of a Strong, Long-Term, Shared Secret
An alternative to salting is to store the plain passwords, but only
for a short while. As soon as the first IKE SA is set up between two
peers, the peers exchange nonces and generate a strong shared secret,
based on IKE's SK_d. They now destroy the short password and replace
it with the new secret.
This method has been added to the current protocol as an optional
mechanism.
Authors' Addresses
Dennis Kuegler
Bundesamt fuer Sicherheit in der Informationstechnik (BSI)
Postfach 200363
Bonn 53133
Germany
EMail: dennis.kuegler@bsi.bund.de
Yaron Sheffer
Porticor
EMail: yaronf.ietf@gmail.com
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